PAUNA OF THE ST. JOHN GEOUP. 48 



Dorsal valve with a A'ery thin cardinal edge turned inward at the hinge. Interior ' 

 marked by three diverging ridges, of which the central one is longer than the lateral ones. 

 The central ridge when well preserved appears to extend two-thirds of the length of the 

 shell, and is distinct as far as the middle of the shell, where there are two small muscular 

 impressions ; at the back of the shell, between the mesian and lateral ridges, are the 

 impressions of the adductor muscles. There is a large, shallow, bilobed depression in the 

 anterior half of the shell. The cardinal border is thin and is made more distinct by a pair 

 of small, narrow, transverse pits on each side of the mesian ridge. 



Ventral valve produced at the back into a low rather blunt beak, depressed at the 

 point. In the interior of the ventral valve, there are at the back three ridges, which 

 diverge from the umbo ; of these the mesian ridge, dividing the posterior adductor 

 muscles, is short and weak ; it is forked at the end (where it terminates against the scar 

 of the pedicel muscle ?). The two lateral ridges are longer, being about a quarter of the 

 length of the shell, and are more distinctly outlined by a long, narrow pit on the inner 

 side. The visceral cavity is wide in front and terminates in a long sinus ; some specimens 

 show ill this sinus a muscular callosity (perhaps due to the anterior retractor muscle). 

 The hinge area is narrow and deflected downward in the middle to receive the end of the 

 dorsal valve ; it is crossed by a shallow pedicel groove. 



Sculpture. — The surface is marked by very fine concentric and radiating lines, and 

 there are stronger concentric growth lines at intervals on the surface. 



Site. — Length of the ventral valve, 8J mm. ; width, 10 mm. ; the dorsal valve is one- 

 half of a millimetre shorter than the ventral. 



Horizon and Localiti/. — In the Dictyonema shales (Div. 3 c) Navy Island, St. John 

 Harbour. 



In many respects de Verneuil's description of O. ApoUinis represents the features of 

 the Canadian species, but there are differences. The most notable of these is that our 

 species does not have the broad flat band at the hinge of the dorsal valve (ventral valve 

 of M. de Verneuil) ; on the contrary, the valve is sharp and narrow edged here. Then 

 also the Canadian species is lenticular in form and not flat, as the Russian species is said 

 to be ; this is shown by examples preserved in pyrite. In the Canadian species all the 

 ridges of the interior of the valves, as well as the hinge area, are more delicate than in the 

 Russian. 



In many examples of the St. John species one finds the body of the shell lenticular, 

 while the edges are flattened, and it seems to be quite certain that the edges of this shell 

 were not only thin and sharp, but sometimes flexible and liable to be bent back by pres- 

 sure. It seems also that in some cases there was a flattening of the edges of the valves 

 even in shells which were of a firmer cousistancy. Such a flattening would give rise 

 to the appearance represented by de Verneuil in his fig. 3 d of Plate XIX. ; but this 

 condition is never seen in our species at the hinge of the dorsal, which always stands 

 up with a sharp edge. This part, in fact, juts into the area of the ventral valve, which 

 is made concave to receive it. An instance of a flattened margin is represented at fig. 

 6 d, PI. XII. 



' I have not found very satisfactory examples of tlie markings of the interior of the valves in any one indi- 

 vidual ; but by combining the markings found on several valves of the dorsal and ventral sides re.spectively it has 

 been found possible to restore some of the features of the interior of the shell. 



